In the case of electrostatic filters for dust separation in industrial waste gases which operate according to the so-called Cottrell principle and which can also be designated as electrostatic separating apparatuses, the changing and transport of the particles to be separated and their depositing on optionally specially formed collecting electrodes occur simultaneously in an electric field, with the particles, after sufficient accumulation or agglomeration, being removed from the collecting electrode either by mechanical vibration (dry dedusting) or by rinsing (wet dedusting). If necessary, several of the aforementioned electric fields are switched in series or also in parallel in order to achieve the desired total separating output.
The reason that some particles are difficult to separate may be caused by the electric properties of the particles which as a result of their chemical and physical properties lead to an insulating layer on the collecting electrode. An additional factor is that as a result of the electric flow turbulence or the so-called electric wind at high current density the share of particles in the grain range<10 μm can be deposited only with more difficulty on the collecting electrode in the region between the charging and the separating electrodes as a result of gas ionization. It is known that as a result of the physically effective charging mechanisms, namely the so-called surge or field charging and the diffusion charging, a more or less strong minimum of particle fraction separating performance occurs. In order to counteract the problems of electric flow turbulence as a result of electric wind, so-called two-stage electrostatic filters were developed, in which the charging and the separating of the particles occur in successively switched separated electric fields. The problematic aspect is the necessary spatial separation of the stages and their different electric high-voltage supply.
In order to solve this problem, an electrostatically working separator for dedusting industrial waste gases is known which works with a negative corona system. A method is known in particular in which particles difficult to separate are removed from a gas fluid with the help of electrostatic charging and separating by means of only one high-voltage source for a high-voltage zone. In contrast to the so-called Cottrell electrostatic filters, the particles are ionized successively and not simultaneously within the high-voltage zone and are thus separated. Moreover, a larger geometrical duct distance is in the ionization zone in this method and in these apparatuses than in the separating zone, as a result of which the field strength of the ionization zone is lower than the field strength of the separating zone. It has thus been managed to substantially avoid the disadvantages of electric turbulence. The duct width for the ionization zone was therefore provided with a larger arrangement in the known filters than the duct width of the separating zone because it was expected that in the comparatively small charging or ionization zone the expected very high, but necessary specific current flow would lead to a relatively early occurring sparkover activity which thus would limit the electric power. This means therefore that in the known filters an ionization zone is associated with at least two, but frequently even three and more separating zones. The presence of a sufficiently high field strength is ensured in the separating zone by the reduction of the duct width.
Similarly working air filters are known from the state of the art, even for purifying breathing air, which are used especially in households, restaurants and lecture halls. Air filters and industrial filters cannot be compared with each other because air filters need to fulfill completely different preconditions than the large industrial electrostatic filters which are concerned here. As a result, they cannot be used for cleaning industrial gases. Duct widths of 200 to 500 mm are usually used in electrostatic filters for industrial waste gases for example. This leads to the consequence, in combination with the composition of the exhaust gases and their flue-gas temperature, that field strengths in the range of 2 to 4 kV/cm and specific currents in the range of 0.2 to 1.2 mA/m2 are usually used in industrial electrostatic filters.
Air filters are also known which each work with a positive corona system and two high voltages. These air filters comprise a rectifier with two outputs for ionization and separation. The field strengths in the known air filters are the same in the ionization and separation zone, but are provided with different voltage potentials. Both zones must be provided with a configuration so as to electrically insulated from each other. Moreover, positive discharge electrodes are provided in the known filters in the ionization zone, which electrodes produce a moderate ionization.
Before the background of ever rising demands placed on the energy efficiency of industrial appliances, the invention is based on the object of avoiding the disadvantages of the described electrostatic filters and electrostatic filter methods and to reduce the energy input in the filtering of industrial waste gases.